Okay this one is buggy. You have to download it from the steam stats page over here. Just click the free button if you already have Hitman installed and it will be added to your library.

Thanks!
Hitman is in my lib, just not installed, and I could not get the pack. So I did as you said: click the free button.
Steam threw an error saying that it could not configure the game and nothing happened -apparently- but a moment later the DLC was correctly listed among those I own.

Yesss!!
Playing Multiplayer, the person who hosts shares their DLC out to all the players.
So if you were to play with, say, me hosting. Then you would be able to play with ALL the DLCs without paying for anything. :V